The present invention relates to a dot line printer in which dot line impressions are carried out during reciprocal movement of a hammer bank which secures a plurality of dot printing hammers. More particularly, the invention relates to such a dot line printer having at least two printing modes different from each other, which is capable of selectively performing dot printings under at least ordinary printing mode and a draft printing mode.
Generally, the dot line printer produces a dot line image during one shuttling movement of the hammer bank, and a plurality of the dot lines will produce one character line during the reciprocal movement of the hammer bank. Throughout the specification and claims, the terms "shuttling direction" imply a reciprocal moving direction of the hammer bank or one dot line extending direction, and the terms "line to line direction" imply a sheet feeding direction or a direction of an array of character lines.
In accordance with the development of data processing techniques, high printing speed and printing quality are required in a dot impact type line printer, which is one typical data output device. However, the requirement of high speed printing is in direct conflict with the requirement of high quality image printing, and thus it is very difficult to satisfy the both requirements at one time.
Further, depending on a processing mode, only one of the requirements may be needed. For example, in case of an ordinary office work, ordinary printing speed is available with a standardized ordinary dot density. On the other hand, if "draft printing" is to be performed, high speed printing is achieved by lowering the dot density. Further, if high dot image quality printing is to be performed, the printing speed must be decreased. This is a conventional dot line printer, a plurality of dot density modes can be set, and a selected one of the dot density modes is used for achieving an intended dot printing speed or print imaging quality. Such conventional dot line printers will be described below with reference to FIG. 1.
In the conventional dot line printer shown in FIG. 1, an ordinary dot density is 160 dpi (dots per inch) in the shuttling direction and 168 dpi in the line to line direction, and 6 dot lines are simultaneously printed during one shuttling motion of a hammer bank 8. The hammer bank 8 is provided with a plurality of printing hammers 16 arrayed in the shutting direction, and one end is connected to a shuttle mechanism S. The shuttle mechanism S generally includes a shuttle motor 1 such as a DC servo motor and a cam mechanism C which includes a shuttle cam 2 coupled to an output shaft of the shuttle motor 1, a shift plate 4 connected to the hammer bank 8, and a pair of cam followers 5 rotatably supported on the shift plate 4. The shuttle cam 2 is in rolling contact with the cam followers 5, and the shuttle cam 2 is eccentrically coupled to the output shaft of the shuttle motor 1, Therefore, the hammer bank 8 is bidirectionally moved in the shuttling direction by the single rotation of the shuttle motor 1 through the cam mechanism C.
The shuttle motor 1 is connected to a motor driver 25 connected to a control circuit 24. Further, a rotary encoder 26 is provided on the output shaft of the shuttle motor 1, and a sensor 27 connected to the control circuit 24 is positioned over the rotary encoder 26.
A platen 20 extends in the shutting direction and is spaced away from the hammer bank 8 by a predetermined gap. A printing sheet 9 is positioned between the platen 20 and the hammer bank 8. Further, an ink ribbon 29 is positioned between the printing sheet 9 and the hammer bank 8. The platen 20 bears dot impression force from the printing hammers 16, and serves to guide a travel of the printing sheet 9. The ink ribbon 29 is moved along an ink ribbon path defined by ribbon guides 23, 23, and is driven by a pair of ink ribbon drive rollers 21, so that the ink ribbon can be foldedly or corrugatedly accommodated in an ink ribbon cassette 22.
The shuttle motor 4 is driven for rotating the shuttle cam 2 by the motor driver 25 controlled by the control circuit 24. The rotation speed of the shuttle motor 1 is detected by the sensor 27 through the rotary encoder 26. A detection signal indicative of the motor rotation speed is transmitted to the control circuit 24 for a feed back control so as to provide a controlled constant rotation speed of the shuttle motor 1.
FIGS. 2 and 3 show a more detailed arrangement of the conventional shuttle mechanism S and a sheet feed mechanism F. In FIG. 2, a U-shaped shuttle frame 7 is disposed around the shift plate 4, and bearings 6 are provided at arm end portions of the shuttle frame 7. Through the bearings 6, 6, a shift shaft 3 axially movably extends, which is connected to the one end of the hammer bank 8. Another end of the hammer bank 8 is fixed to a bank shaft 17 which is slidingly supported by a shuttle bearing 19 fixed on a bank shaft holder 18.
The sheet feed mechanism F includes a sheet feed motor 10 fixedly secured to a side frame 30. A motor shaft of the sheet fed motor 10 extends through the side frame 30 and a drive pulley 11 is coupled to the motor shaft. A drive shaft 14 is rotatably supported by the side frames 30, and a driven pulley 13 is coupled to one end of the drive shaft 14. An endless belt 12 is mounted on the drive and the driven pulleys 11 and 13 for rotating the drive shaft 14 about its axis. A tractor 15 is mounted on the drive pulley 14 for feeding the printing sheet 9 in the sheet feeding direction, i.e, line to line direction.
In FIG. 3, the printing sheet 9 is intermittently fed in the line to line direction as indicated by an arrow Y when no dot printing is carried out to the printing sheet, i.e., when the hammer bank 8 is moved to a hammer bank reverse moving region. More specifically, by means of the shuttle mechanism S, the hammer bank 8 repeatedly performs reciprocal movement in the shuttling direction indicated by an arrow X in FIG. 3 in accordance with a cam profile of the shuttle cam 2 which defines a cam lift characteristic as shown in FIG. 4. In the cam lift characteristic shown in the graph of FIG. 4, in a printing regions P1 and P2, the hammer bank 8 is moved at a constant velocity, and in two reverse moving regions R1 and R2, the hammer bank speed is changed for reversibly changing the moving direction of the hammer bank 8. In the two reverse moving regions R1 and R2, the printing sheet 9 is fed by a predetermined dot number pitches by means of the above described sheet feed mechanism F. Thus, the dot printing in one shuttling direction is achieved without the sheet feed in the P1 region, and when the hammer bank 8 is reversely moved in the reverse moving region (R2) the sheet feeding with the predetermined length is performed, and thereafter, the dot line printing in an opposite shuttling direction is performed in the P2 region.
In such conventional dot line printers, in order to increase printing speed, the draft printing mode is selected in which dot density in the shuttling direction is lowered, and a rotation speed of the shuttle motor 1 is increased to increase moving velocity of the hammer bank 8. In order to maintain a given printing quality, the reverse moving period , i.e., the sheet feeding period in the draft printing mode must be equal to the period in case of the ordinary printing mode. However, the lift characteristic of the shuttle cam 2 of the conventional dot printer is designed to meet with the ordinary printing mode. Therefore, if the rotation speed of the shuttle motor 1 is increased for the draft printing, reverse period of the hammer bank 8 is also shortened. To make the reverse period in the draft mode equal to that in the ordinary printing mode, rotation speed of the shuttle motor 1 is reduced at the time of the reverse period in the draft printing mode. This will be explained with reference to FIGS. 5 and 6.
FIG. 5 shows the relationship between an angular velocity .omega. of the shuttle motor 1 in the reverse and the printing/periods of the hammer bank 8. A line I represents the angular velocity .omega..sub.1 of the shuttle motor 1 under the ordinary printing mode in one moving direction of the hammer bank 8, and a line II represents the angular velocity .omega..sub.2 of the shuttle motor 1 under the draft mode in the one moving direction of the hammer bank 8. When using the shuttle cam 2 which provides the reversing period and the printing period of the hammer bank 8 such as those shown in the line I in the ordinary printing mode at the angular velocity .omega..sub.1, the reverse period would be reduced if the angular velocity of the shuttle motor 1 is increased to .omega..sub.2.
In order to obviate this problem, and to make the reverse period in the draft mode equal to that in the ordinary printing mode, angular velocity of the shuttle motor 1 must be controlled at the reverse period of the hammer bank 8 in the draft printing mode as shown in FIG. 6. That is, when the hammer bank 8 is moved at a transitional position from the printing position to the reversely moving position in the draft printing mode, the angular velocity .omega..sub.2 at the time of the printing period is suddenly reduced to (2.omega..sub.1 -.omega..sub.2), and when the hammer bank 8 is moved from the reversely moving position to the printing position, the angular velocity (2.omega..sub.1 -.omega..sub.2) is rapidly increased to .omega..sub.2. That is, deceleration and acceleration of the shuttle motor 1 is repeatedly performed at the reversing period. Accordingly, a large shuttle motor 1 must be used so as to provide high output capable of providing a maximum output torque of EQU 2I(.omega..sub.2 -.omega..sub.1)/t.sub.F
where I designates a moment of inertia of the output shaft of the shuttle motor, and t.sub.F designates a half period of the sheet feeding period.